Abstract

Sequence analysis of the Ecca—Beaufort boundary in the southern Karoo Basin has revealed three separate facies associations spanning the stratigraphic interval between the top of the Fort Brown Formation and the lowermost maroon mudrocks of the Beaufort Group. This sequence was deposited in prodelta, delta front, and delta plain environments respectively. The lithological contact between the rocks deposited in the delta front and delta plain represents the palaeoshoreline, and occurs only once in the stratigraphic sequence, suggesting a continuous normal regression from Ecca to Beaufort times. This diachronous shoreline is associated with deltaic progradation within ahighstand systems tract. Sediment deposition was mainly a result of ephemeral flash floods, but perennial rivers also flowed from melting ice-capped highlands to the palaeosouth. Two separate fossil associations have been recognized and correlate with the lithological subdivisions. The lower fossil assemblage occurs in the Fort Brown andWaterford formationsand is characterized by silicified wood, comminuted plant material, and fish scales. The upper association occurs only in the lower Beaufort Group and includes in situ equisitalean, and well-preserved Glossopteris plant fossils and tetrapods of the Eodicynodon Assemblage Zone. The Beaufort—Ecca boundary coincides with the position of the palaeoshoreline. This lithological contact also marks achange in depositional style and palaeontological character between the two groups. The new placementof the boundary is some 300— 650m below the presently mapped contact in the south of the basin.

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